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1.3 PHYSICAL AND HYDROGEOLOGIC SETTING The Site is situated at the southern end of the Sacramento Valley, a subregion of the Central Valley geomorphic province. The Sacramento Valley is a large asymmetric trough that is bounded by granitic, metamorphic, and marine sedimentary rocks of pre-Tertiary age. The trough has been filled with as much as 10 vertical miles of sediment ranging in age from Jurassic to Holocene. Topography in the site vicinity is relatively flat with a very gentle downward slope towards the southwest. The Site lies at an approximate elevation of 45 feet above mean sea level. The nearest surface water body is the Mokelumne River, located approximately 0.75 miles north of the Site. According to a geologic map published by the United States Geological Survey(U.S.G.S., 1986), the I site vicinity is underlain by Pliocene to Holocene continental deposits. Lithologic information from soil borings advanced by SEACOR at the Site indicate that the upper 50 feet of soils consist of unconsolidated alluvium composed predominantly of fine sand with lesser and variable amounts of silt, clay, and gravel interbedded with finer grained silt and sandy clay lenses. Groundwater level measurements have been taken on a quarterly basis in well MW-1A since May 1991. Depth to groundwater in well MW-1A has ranged from 44.5 feet to 47 feet below ground surface. According to a water elevation map prepared by the San Joaquin County Flood Control and Water Conservation District, regional groundwater flow direction is towards the south. 1.4 OBJECTIVES AND SCOPE OF WORK SEACOR conducted this investigation in general accordance with our Work Plan to Conduct Groundwater Investigation and Confirmatory Soil Sampling (Work Plan), dated December 18, 1992 (SEACOR, 1992). SEACOR submitted the Work Plan to the SJPHS prior to initiating field work. Ms. Mary Meays of the SJPHS provided SEACOR with a verbal conditional approval to proceed. ' Ms. Meays requested that a minimum of two soil samples from each monitoring well location be chemically analyzed, regardless of field screening results. Ms. Meays requested that these soil samples be collected from depths corresponding to the high water table level (approximately 30 feet) ' and directly above the current water table level (approximately 45 feet). The objectives of SEACOR's groundwater investigation were: (1) to confirm the anticipated direction and gradient of groundwater flow beneath the Site; and (2) to evaluate groundwater quality downgradient from the former USTs. The specific tasks performed by SEACOR for the groundwater investigation included: Installing two additional groundwater monitoring wells in the anticipated ' downgradient direction from the former USTs. Chemically analyzing two soil samples from each borehole. Surveying the top of casing elevation of the newly-installed wells and existing well and . sounding the depth to water in each well. i' ' MARDEE.RPT 50014-003-01 MD04 3